Apparatus and method for reallocating logical to physical disk devices using a storage controller with access frequency and sequential access ratio calculations and display

ABSTRACT

A storage controller calculates an access frequency of each logical disk; that is selects a first logical disk device of which the access frequency exceeds a first predetermined value, the first logical disk device being allocated to a first physical disk device; selects a second logical disk device which has the access frequency equal to or less than a second predetermined value, the second logical disk device being allocated to a second physical disk device; and reallocates the first and second logical device; and reallocates the first and second logical devices to the second and the first physical disk device, respectively.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.11/405,544, filed Apr. 18, 2006 (now U.S. Pat. No. 7,577,815), which isa continuation of U.S. application Ser. No. 11/142,458, filed Jun. 2,2005, now U.S. Pat. No. 7,069,383, which is a continuation ofapplication Ser. No. 10/684,453, filed Oct. 15, 2003, now U.S. Pat. No.6,915,382; which is a continuation of application Ser. No. 10/112,865,filed Apr. 2, 2002, now U.S. Pat. No. 6,708,252; which is a continuationof application Ser. No. 09/335,175, filed Jun. 17, 1999, now U.S. Pat.No. 6,446,161; which is a continuation of application Ser. No.08/833,347, filed Apr. 4, 1997, now U.S. Pat. No. 5,956,750, thecontents of which are incorporated herein by reference.

This application relates to a U.S. patent application Ser. No.08/824,308, filed Mar. 27, 1999, now abandoned, entitled “STORAGESYSTEM” by Akira YAMAMOTO et al., and assigned to the present assignee,based on Japanese Patent Application No. 8-72934, disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of, and an apparatus for,reallocating logical disk devices to physical disk devices using astorage controller, and in particular, to a method of, and an apparatusfor, reallocating logical disk devices to physical disk devices using astorage controller for improving data access performance.

A technology related to data arrangement on a disk array has beendisclosed in pages 109 to 116 of an article “A Case For Redundant ArraysOf Inexpensive Disks (RAID)” reported by D. Patterson, G. Gibson, andRandy H. Katz in ACM SIGMOD conference held at the University ofIllinois, Chicago in June 1988. In this article, the disk arrays areclassified into five levels including RAID1 to RAID5.

In a disk array of RAID1, write data from a data processor unit iswritten in a main disk device and a copy thereof is stored in a sub-diskdevice called a mirror, thereby guaranteeing reliability of data inRAID1. In RAID1, since check information is a copy of the original data,only a small overhead is required to generate the check information andhence a desirable access performance can be obtained. However, the usageefficiency of physical storage devices is reduced to, for example, 50%.

Additionally, in the disk array of RAID5, check information calledparity is produced for a plurality of write data items from the dataprocessor unit. In RAID5, in a parity creation to generate a new parityitem, it is necessary to read data and parity respectively generatedbefore an update operation. Therefore, lengthy overhead is required forthe generation of check information and hence the access performance isreduced. However, only one parity item is created for a plurality ofdata items, the usage efficiency of storage devices is higher whencompared with that of storage devices of RAID1.

Furthermore, in JP-A-7-84732 (US Priority 109,137 (Aug. 19, 1993), therehas been disclosed a method in which disk devices are subdivided intoRAID1 and RAID5 areas such that data is moved between RAID1 and RAID5according to a performance protocol.

Moreover, in a report “Performance Evaluation Of Hot Mirrored DiskArrays On Disk Failure” written by K. Mogi and M. Kitsuregawa in pages19 to 24 of the Technical Report of IEICE, CPSY95-82, DE95-68 (1995-12),Vol. 95-No. 407, there has been disclosed a technology in which a RAIDlevel of data is dynamically changed according to an access frequency.Specifically, disk devices are subdivided into RAID1 and RAID5 areassuch that data for which write access is requested is preferentiallystored in the RAID1 area. With this provision, data having a high accessfrequency can be stored in the RAID1 area and data having a low accessfrequency can be stored in the RAID5 area.

As a result of, this technology, physical disk devices having mutuallydifferent values of storage capacity and physical disk devices havingmutually different RAID levels can be installed in a mixed fashion inthe storage subsystem. Furthermore, data in a logical disk device can bestored in arbitrary logical disk devices according to indices such asthe access frequency and pattern thereof. In addition, when storing datahaving a high access frequency, it may also be possible to alter thestorage position thereof so that the data is written in a physical diskdevice having a higher speed.

According to the above, the data storage position is varied in the unitof data to be accessed. Therefore, successive data items in a logicaldisk device which is directly accessed by the data processing unit arediscontinuous in physical disk devices in which the data items areactually stored.

On the other hand, according to the report “DE95-68” above, each time awrite request is issued, data regarded to have a low access frequency ismoved from the RAID1 area the RAID5 area so as to write the data in anavailable area thus reserved in the RAID1. Consequently, when the accessfrequency is low for an access pattern in a random access operation,most of data items moved to the RAID1 area are returned again to theRAIDS area.

SUMMARY OF THE INVENTION

It is therefore a first object of the present invention to reallocatelogical disk devices having a high access frequency to physical diskdevices having a higher speed.

According to a first aspect of the present invention, each of aplurality of logical disk devices are consecutively arranged in theplural physical disk devices. Next, the access frequency is calculatedfor each logical disk device. Thereafter, according to results of thecalculation, there ‘is selected a first logical disk device of which theaccess frequency exceeds a first predetermined value, the first logicaldisk device being allocated to a first physical disk device. There isfurther selected a second logical disk’ which is allocated to a secondphysical disk device which has an operation speed higher than that ofthe first physical disk device and which has the access frequency equalto or less than a second predetermined value. The first and secondlogical disk devices are allocated to the second and first physical diskdevices, respectively.

Moreover, a second object of the present invention is to reallocatelogical disk devices having a high sequential access ratio to physicaldisk devices having a higher performance of sequential access.

According to the second aspect of the present invention, each of theplural logical disk devices is successively arranged in the pluralphysical disk devices. Subsequently, the ratio of sequential access iscalculated for each logical disk device. Thereafter, according toresults of the calculation, there is selected a first logical disk ofwhich the sequential access ratio exceeds a first predetermined value,the first logical disk device being allocated to a first physical diskdevice. Furthermore, there is selected a second logical disk devicewhich is allocated to a second physical disk device capable of achievinga sequential access at a speed higher than that of the first physicaldisk device and which has a sequential access ratio equal to or lessthan a second predetermined value. The contents of the first physicaldisk device are then exchanged with those of the second physical diskdevice to thereby reallocate the first and second logical disk devicesto the second and first physical disk devices, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome apparent with reference to the following description andaccompanying drawings wherein:

FIG. 1 is a block diagram of an information processing system includinga storage controller related to a first embodiment of the presentinvention;

FIG. 2 is an explanatory diagram showing a correspondence relationshipbetween logical and physical disk devices;

FIG. 3 is a diagram showing the layout of logical-physical devicemapping information;

FIG. 4 is a diagram showing the layout of logical disk information;

FIG. 5 is diagram showing the layout of information of data accessing;

FIG. 6 is a block diagram showing operation of the storage controller inthe first embodiment of the present invention;

FIG. 7 is a flowchart of a process part of reallocation of a logicaldisk device;

FIG. 8 is a flowchart of a process part for calculating access positionof a physical disk device;

FIG. 9 is a block diagram showing operation of the storage controller inthe third embodiment of the present invention; and

FIG. 10 is a flowchart of a process part to determine the necessity, orlack thereof, of reallocation of logical disk device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

In the first embodiment, information of data accessing of each logicaldisk device is collected by the storage controller such that theinformation is notified via the service processor (SVP) to theserviceperson. According to the information, the serviceperson issues areallocation indication, which causes the logical disk devices to bereallocated to the physical disk devices.

FIG. 1 is a block diagram of an information processing system includingthe storage controller in a first embodiment of the present invention.The information processing system 1 includes a data processor unit 100,a storage controller 104, at least one physical disk device 105, and aservice processor SVP 111 which are connected to each other.

The data processor unit 100 includes a, central processing unit (CPU)101, a main storage 102, and channels 103.

The storage controller 104 includes at least one director 106, a cachememory 107, a directory 108, a non-volatile memory 109, non-volatilememory control information 110, logical-physical device mappinginformation 300, logical disk device information 400, and information ofdata accessing 500.

The director 106 conducts transfers of data between the channels 103 ofthe data processor unit 100 and the physical disk devices 105, betweenthe channels 103 of the data processor unit 100 and the cache memory107, and the cache memory 107 and the physical disk devices 105.

Loaded in the cache memory 107 is data having a high access frequency inthe physical disk devices 105. The load process is executed by thedirector 106. The data to be loaded therein specifically includes accessobjective data to be accessed by the CPU 101 of the data processor unit100 and data in the neighborhood of the access objective data in thephysical disk devices 105.

The directory 108 stores therein information for controlling the cachememory 107.

Like the cache memory 107, the non-volatile memory 109 stores thereindata having a high access frequency in the physical disk devices 105.

The non-volatile memory control information 110 is used to store thereininformation for controlling the non-volatile memory 109.

The logical-physical device mapping information 300 is informationindicating positions of the respective logical disks (200 in FIG. 2which will be described later) allocated to the physical disk devices105 and the logical disk devices (200 in FIG. 2) located in therespective physical disk devices. Utilizing the information, a storagearea in the physical disk devices 105 is calculated for access objectivedata of the CPU 101 of data processor unit 100. The logical disk deviceinformation 400 indicates states such as accessibility ornon-accessibility of each logical disk device (200 in FIG. 2). Thelogical-physical device mapping information 300 and logical diskinformation 400 are recorded on a non-volatile media so that theinformation items are kept retained therein, for example, even when thepower source is interrupted.

The information of data accessing 500 stores information such as anaccess frequency and an access pattern for each logical disk device (200in FIG. 2). Each of the physical disk devices 105 includes a media onwhich data is recorded and a device for reading or writing the recorddata from or on the media.

The SVP 111 presents the information of data accessing 500 to theserviceperson and receives an input of indication for reallocation fromthe serviceperson. Moreover, the SVP 111 transmits an indication fromthe serviceperson to the information processing system 1 as well asnotifies a state of defect or the like of the information processingsystem 1 to the serviceperson.

FIG. 2 is a diagram showing a relationship between the logical diskdevices 200 and the physical disk devices 105.

The logical disk devices 200 are virtual disk devices to be directlyaccessed by the CPU 101 and are associated with the physical diskdevices 105 in which access objective data is actually stored. Data isconsecutively arranged in the physical disk devices 105 in considerationof the sequential access. When the physical disk devices 105 to whichthe data of the data processor unit 100 is allocated are in a disk arrayconfiguration, the logical disk device 200 corresponds to a plurality ofphysical disk devices 105 as shown in FIG. 2. Moreover, when thecapacity of physical disk device 105 is larger than that of the logicaldisk device 200 and data of a plurality of logical disk devices can bestored in one physical disk device 105, the physical disk device 105corresponds to a plurality of logical disk devices 200. Thecorrespondence between the physical disk devices 105 and the logicaldisk devices 200 is controlled by the logical-physical device mappinginformation 300. For example, when the CPU 101 reads data 201 from thelogical disk device 200, the storage controller 104 obtains the physicaldisk device 105 corresponding to the logical disk device 200 accordingto the logical-physical device mapping information 300. Moreover thecontroller 104 attains a data storage position 202 in the area of thephysical disk device 105 to conduct a data transfer operation.

FIG. 3 is a diagram showing the logical-physical device mappinginformation 300.

The mapping information 300 includes logical disk layout information 310and physical disk layout information 320. The logical disk layoutinformation 310 is information related to the area in the physical diskdevice 105 to which each logical disk device 200 is allocated. Theinformation 310 is adopted to obtain, according to a logical disk device200, a physical disk device 105 corresponding thereto. On the other handthe physical disk layout information 320 includes information related tothe logical disk device 105 allocated to each physical disk device 200and is used to obtain, according to a physical disk device 105, alogical disk device 200 corresponding thereto.

The logical disk layout information 310 includes, for each of thelogical disk devices 200, a set of a physical disk device group 311, aRAID configuration 312, and a start position 313. The physical diskdevice group 311 is information denoting a physical disk device to whichthe pertinent logical disk device is allocated. The RAID configuration312 designates a RAID level of the physical disk device group 311. Thestart position 313 indicates the start position in the physical diskdevice 105 of the pertinent logical disk device 200 allocated to thephysical disk device 105.

The physical disk layout information 320 includes a logical disk devicegroup 321 for each of the physical disk devices 105. The group 321indicates the logical disk device 200 allocated to the pertinentphysical disk device 105.

FIG. 4 is a diagram showing the logical disk information 400.

The information 400 includes for each of the logical disk devices 200 alogical disk state 401 and a reallocation pointer 402. The state 401denotes the state of logical disk device 200 such as “normal” blockade”,“formatting in process”, or “reallocation in process”. The pointer 402is information effective only when the logical disk state 401 is“reallocation in process” and designates a position next to the area ofthe logical disk device already reallocated, namely, the first positionof the area thereof not yet reallocated. In a data access thereto by theCPY 101 during “reallocation in process”, when the access is issued toan area of which the address is smaller than that of the reallocationpointer 402, the access is made to the physical disk device 105 afterthe reallocation. On the other hand, when the access is issued to anarea of which the address is larger than that of the reallocationpointer 402, the access is made to the physical disk device 105 beforethe reallocation. The access operation will be described in detaillayer.

FIG. 5 shows the access information 500.

The information 500 includes access frequency information 501 and accesspattern information 502 for each of the logical disk devices 200. Thisinformation can be referred to from the data processor unit 100 as wellas the SVP 111. The access frequency information 501 keeps the number ofaccesses to the logical disk device 200 per unit of time. Thisinformation 501 is adopted as an index to attain any logical disk device200 having a higher or lower access frequency among the logical diskdevices 200. The access pattern information 502 keeps the ratio betweenthe sequential accesses and the random accesses to the pertinent logicaldisk device 200. This information 502 is employed as an index to obtainone of the logical disk devices 200 desirably reallocated to thephysical disk devices 105 for which sequential accesses are frequentlyconducted and which has a high sequential performance.

FIG. 6 is a diagram showing in detail the operation of the storagecontroller 104.

First, description will be given of operation in the read and writeprocesses by the storage controller 400.

When executing a read or write process, the director 106 ordinarilyreceives an indication of CPU 600 from the CPU 101 via the channel 103.The CPU indication 600 includes specifying information 1 designating alogical disk device 200 in which a record to be read (or written) isstored and specifying information 2 designating a position (track,sector, record) in the logical disk device 200 in which the record to beread (or written) is stored.

In a calculation process of access position (610), the director 106calculates an access position in the physical disk device 105 using theindication 600 from the CPU 101 and the logical-physical device mappinginformation 300. The calculation process (610) will be described indetail later by referring to FIG. 8.

Thereafter, for example, in the read process, the data at the calculateddata storage position 202 in the physical disk device 105 is read to bemoved as data 201 in the cache memory 107. The obtained data 201 istransferred via the channel 103 to the main storage 102.

Subsequently, a process of collecting access information 500 will bedescribed.

In the access for a read/write process by the CPU 101, the director 106updates the access information 500 of the logical disk device 200 as theaccess object. The collection of access frequency information 501 isaccomplished, for example, such that an internal counter increments itsvalue for each access. After a lapse of a predetermined period of timeor after a predetermined number of accesses are conducted, when anaccess is issued, the access frequency acquired from the internalcounter is checked. The collection of access pattern information 502 iscarried out, for example, such that an internal counter increments itsvalue for each sequential accesses. After a lapse of a predeterminedperiod of time or after a predetermined number of access are conducted,when an access is issued, the access pattern acquired from the internalcounter is checked.

Next, the indication of reallocation 620 will be described. Referring tothe information of data accessing 500 presented .via the SVP 111, theserviceperson determines the necessity of reallocation for each logicaldisk device. The contents of the decision made by the serviceperson arethe same as those of a decision regarding the necessity, or lack thereofof reallocation for logical disk devices (910), which will be laterdescribed in conjunction with the third embodiment by referring to FIG.10. When there exists a logical disk device 200 decided for allocationas a result of the decision, the serviceperson issues an indication ofreallocation 620 via the SVP 111 to the storage controller 104.

The indication of reallocation 620 includes information of indication1-2 specifying two logical disk devices 200 as objects of reallocation.

Subsequently, the process of reallocation of logical disk devices (630)will be described. FIG. 7 is a process flowchart of the part of processof reallocation of logical disk devices (630). On receiving theindication of reallocation 620, the director 106 carries out the processof reallocation between the specified logical disk devices 200 (630).

First, in step 700, the state of logical disk 401 of each of thespecified logical disk devices 200 is set to “reallocation in process”in the logical disk information 400.

In step 701, the reallocation pointer 402 of each of the specifiedlogical disk devices 200 is set to the first position of each associatedlogical disk device 200 in the logical disk information 400.

In step 702, the reallocation pointer 402 of each of the specifiedlogical disk devices 200 is checked in the logical disk information 400.If the reallocation has not been completed for the entire region,control is passed to step 703. If the reallocation has been completed,control is transferred to step 707.

In step 703, data of one processing unit of reallocation is transferredfrom the physical disk device 105 to the cache memory 107 beginning at adata position designated by the reallocation pointer 402. In thisoperation, the data of one processing unit of reallocation is set to theleast common multiple of the respective amounts of data respectivelycorresponding to check information items of two logical disk devices 200as the objects of reallocation. For example, assume that thereallocation is accomplished between a logical disk device 200 of RAID5and a logical disk device 200 of RAID1. Since the amount of data for onecheck information of the disk device 200 of RAID1 is “1”, the amount ofdata for one processing unit is equal to that of the amount of datacorresponding to one check information item of the disk device 200 ofRAID 5, namely, equal to that of one parity item.

In step 704, when the logical disk device 200 as the destination ofreallocation is at the RAID level having parity, the parity is generatedfor the data 201 of one processing unit in the objective data ofreallocation of the cache memory 107.

In step 705, the data 201 of one processing unit in the objective dataof reallocation of the cache memory 107 and the parity produced in step704 above are written in the physical disk device 105 as the destinationof reallocation.

In step 706, the reallocation pointer 402 is advanced according to oneprocessing unit, and then control is returned to step 702.

Incidentally, the data and parity are also transferred to thenon-volatile memory 109 in steps 703 and 704 such that these items areretained even at occurrence of, a failure in the cache memory for thefollowing reasons. Assume in the writing operation in step 705 in which,for example, data of the first and second logical disk devices 200 areto be processed. When data of the first logical disk device 200 iswritten in the logical disk device 105, if a failure occurs in the cachememory 107 and hence data thereof cannot be accessed, the data in thesecond logical disk device 200 for which the write operation is notcompletely achieved will be lost.

In step 707, the logical-physical mapping information 300 is updated.Namely, the logical disk layout information 310 and the physical disklayout information 321 are modified.

In step 708, the logical disk state 401 of logical disk information 400is restored to the normal state and then the reallocation process (630)is terminated.

FIG. 8 is a processing flowchart of the calculation process 610.

In step 800, a check is made to determine whether or not the logicaldisk state 401 of the logical disk device 200 as the object of access is“reallocation in process” in the logical disk information 400. If thisis the case, control is passed to step 801; otherwise, control istransferred to step 803.

In step 801, the access data position is compared with the reallocationpointer 402 of the logical disk device 200 as the object of access inthe logical disk information. When the access data position is largerthan the position indicated by the pointer 402, control is passed tostep 802; otherwise, control is transferred to step 803.

In step 802, the logical disk device 200 as the destination ofreallocation of the pertinent logical disk device 200 is set as theobject of access. Thereafter, control is passed to step 804.

In step 803, the pertinent logical disk device 200 is set as the objectof access.

In step 804, according to the logical-physical mapping information 300,there is calculated an access position in the physical disk device 105corresponding to the logical disk device 200 as the object of access.

According to the information processing system 1 and the storagecontroller 104 related to the first embodiment, logical disk devicehaving a high access frequency can be reallocated to physical diskdevices having a higher speed according to decision of the servicepersonbased on the information of data accessing 500. Additionally, logicaldisk devices having a high ratio of sequential access can be reallocatedto physical disk devices having a higher performance of sequentialaccess. Consequently, the access performance can be improved.

Second Embodiment

The first embodiment may be modified such that the storage controller104 notifies that the information of data accessing 500 is presented tothe data processor unit 100, which then determines the necessity or lackthereof of reallocation to issue an indication of reallocation(equivalent to the indication of reallocation 620).

Third Embodiment

In the third embodiment, the indication of reallocation is not receivedfrom the SVP 111 and the data processor unit 100. Namely, the indicationof reallocation is determined by the storage controller 104.

FIG. 9 is a diagram showing in detail the operation of indication ofreallocation by the storage controller 104.

This differs from the first embodiment (FIG. 6) in that the process partof determining the necessity, or lack thereof of reallocation of logicaldisk 910 issues the indication of reallocation 620.

FIG. 10 is a process flowchart of the process part 910.

This process (910) is executed by the director 106 examining theinformation of data accessing 500 of each logical disk device 200 at afixed interval of time.

In step 1000, referring to the access frequency information 501 ofaccess information 500, a check is made to decide whether or not thereexists any logical disk device 200 (to be referred to as a firstcandidate logical disk device herebelow) for which the access frequencyexceeds a predetermined value and which is allocated to a physical diskdevice 105 having a relatively low speed. If there exists such a logicaldisk device 200, control is passed to step 1001; otherwise, control istransferred to step 1005.

In step 1001, referring to the access pattern information 502 of thefirst candidate logical disk device 200, a check is made to decidewhether or not the sequential access ratio is equal to or more than apredetermined value. If this is the case, control is passed to step1002; otherwise, control is transferred to step 1004.

In step 1002, referring to the access frequency information 501 of alogical disk device 200 allocated to a physical disk device 105 having aspeed higher than that of the first candidate logical disk device 200, acheck is conducted to determine whether or not there exists a logicaldisk device 200 (to be referred to as a second candidate logical diskdevice) of which the access frequency is equal to or less than thepredetermined value. If there exists such a logical disk device, controlis passed to step 1003; otherwise control is transferred to step 1005.

In step 1003, it is decided that the process of reallocation (630) isrequired to be conducted between the first and second candidate logicaldisk devices 200 and the indication of reallocation 620 is issued. Theprocessing is then terminated.

In step 1004, referring to the access pattern information 502 of thelogical disk device 200 allocated to a physical disk device 105 havingsequential performance higher than that of the first candidate logicaldisk device 200, a check is made to decide whether or not there exists alogical disk device 200 (to be referred to as a second logical diskdevice herebelow) having a sequential access ratio equal to or less thanthe predetermined value. If this is the case, control is transferred tostep 1003; otherwise, control is passed to step 1002.

In step 1005, it is determined that the process of reallocation (630) isunnecessary for the logical disk devices 200. The processing is thenterminated. In accordance with the information processing system 1 andthe storage controller 104 related to the third embodiment, logical diskdevice having a high access frequency can be automatically reallocatedto physical disk devices having a higher speed according to theinformation of data accessing 500. Additionally, logical disk deviceshaving a high ratio of sequential access can be reallocated to physicaldisk devices having a higher performance of sequential access.Therefore, the access performance can be improved.

Fourth Embodiment

The first to third embodiments may be modified such that reliabilityrequired for the logical disk devices 200 is employed, in addition to orin place of the information of data accessing 500, as the index fordecision of necessariness or unnecessariness of reallocation. Whenreliability is employed as the index, it is possible to improve the datareliability in the logical disk devices 200.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by thoseembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and spirit of the present invention.

1. A storage system comprising: a plurality of physical disk devicesstoring data of a plurality of logical disk devices, each of saidplurality of logical disk devices is a target device of an accessrequest from a computer coupled to the storage system; and a controllercoupled to said plurality of physical disk devices, wherein, in responseto a request of reallocation, said controller transfers data from atleast one of said plurality of physical disk devices to at least anotherone of said plurality of physical disk devices, wherein if saidcontroller receives said access request from said computer, saidcontroller processes said access request during said reallocation.
 2. Astorage system according to claim 1, further comprising: a memorycoupled to said controller, for storing data from said computer and/orsaid plurality of physical disk devices, wherein, in response to saidrequest of reallocation, said controller transfers and stores data fromsaid at least one of said plurality of physical disk devices to saidmemory, and wherein said controller transfer data in said memory to saidat least another one of said plurality of physical disk devices afterstoring data to said memory.
 3. A storage system according to claim 1,wherein said controller specifies an access position in said pluralityof physical disk devices based on said request of reallocation.
 4. Astorage system according to claim 1, wherein said controller transferssaid data which is set to a least common multiple of amount of data. 5.A storage system according to claim 1, wherein a new parity is generatedif data of said at least another one of said plurality of physical diskdevices includes a parity, wherein said controller writes said datawhich read from said at least one of said plurality of physical diskdevices and said new parity in said at least another one of saidplurality of physical disk devices.
 6. A storage system according toclaim 1, wherein if said controller receives said access request fromsaid computer, said controller determines whether the target device ofsaid plurality of logical disk devices is reallocating or not, whereinif said target device is not reallocating, said controller specifies anaccess position in said plurality of physical disk devices according tomapping information between said plurality of physical disk devices andsaid plurality of logical disk devices, wherein if said target device isreallocating, said controller compares an access data position with areallocation pointer of said target device, wherein if said access dataposition is larger than said reallocation pointer, said controllerspecifies that said target device is a logical disk device correspondingto said at least another one of said physical disk devices.
 7. Areallocation method, used in a storage system having a control unitcoupled to a data processing unit, and a plurality of physical diskdevices to which at least two logical disk devices are allocated,wherein a logical disk device is a virtual disk device which is a targetdevice of an access request from said data processing unit, forreallocating said at least two logical disk devices to a plurality ofphysical devices, said reallocation method performed by said controlunit comprising the steps of: storing data of a plurality of logicaldisk devices to a plurality of physical disk devices; and transferringdata, in response to a request of reallocation, from at least one ofsaid plurality of physical disk devices to at least another one of saidplurality of physical disk devices, wherein if said controller receivessaid access request from said data processing unit, said controllerprocesses said access request during said reallocation.
 8. Areallocation method according to claim 7, further comprising the stepsof: transferring data, in response to said request of reallocation, fromsaid at least one of said plurality of physical disk devices and storingdata to a memory coupled to said controller, wherein said memory storesdata from said computer and/or said plurality of physical disk devices;and transferring data, in response to said request of reallocation, tosaid at least another one of said plurality of physical disk devicesafter storing data to said memory.
 9. A reallocation method according toclaim 7, further comprising the step of: specifying an access positionin said plurality of physical disk devices based on said request ofreallocation.
 10. A reallocation method according to claim 7, whereinsaid transferring step includes transferring said data which is set tothe least common multiple of amounts of data.
 11. A reallocation methodaccording to claim 7, further comprising the steps of: generating a newparity if data of said at least another one of said plurality ofphysical disk devices includes a parity; and writing said data whichread from said at least one of plurality of physical disk devices andsaid new parity in said at least another one of said plurality ofphysical disk devices.
 12. A reallocation method according to claim 7,further comprising the steps of: determining, upon receiving said accessrequest from said computer, whether a target logical disk device of saidplurality of logical disk devices is reallocating or not; specifying, ifsaid target logical disk device is not reallocating, an access positionin said plurality of physical disk devices according to mappinginformation between said plurality of physical disk devices and saidplurality of logical disk devices; and comparing, if said target deviceis reallocating, an access data position with a reallocation pointer ofsaid target logical disk device, and if said access data position islarger than said reallocation pointer, specifying that said targetlogical disk device is a logical disk device corresponding to said atleast another one of said physical disk devices.